The present invention relates to a system for monitoring the tire pressure in mounted vehicle tires by measuring, comparing and evaluating the rotating speeds of the individual wheels, including wheel speed sensors, and an electronic evaluating circuit which determines correction factors for the wheel speeds of the individual wheels representative of the deviation of the respective wheel speed from a reference quantity, the evaluating circuit determining and storing the values of the individual correction factors in a learning phase, and determining and evaluating the deviation of the instantaneous correction factors in consideration of the respective driving situation in an identification phase, and signaling loss of tire pressure as soon as the deviation of the instantaneous values from the learnt values in consideration of the driving situation exceeds a predetermined amount.
A circuit arrangement of this type is disclosed in German patent application No. 43 37 443. The values for the individual correction factors are determined in the learning period. A band width or tolerance width for the correction factors responsive to the respective driving situation is determined by correlations between the correction factors for typical driving situations. Loss of tire pressure is signaled as soon as it is identified in the identification period that the tolerance width is exceeded.
The loss of tire pressure is identified during driving with the prior art device. The operation of the device is based on the knowledge that the air pressure in the tires influences the tire tread circumference. The dependence on pressure is, however, very insignificant and, therefore, difficult to measure. The wheel speed sensors which are already required in a vehicle (for an anti-lock control system, for example) can now be used to conclude a loss of pressure from the travel measurement and the comparison of the travels covered by the four wheels. Because the changes in circumference of the tires can hardly be resolved, not even in the event of a major pressure loss, the sources of disturbances, more particularly, the different tire diameters due to tire tolerances, tire mixing, different wear, or as a consequence of assembly of an emergency wheel, must be eliminated in the signal analysis. In the prior art tire pressure monitoring device, this is effected by a calibration tip switch which the driver or the workshop personnel is required to operate manually, at least after each tire change. The pressure monitoring system described will subsequently be adjusted in the learning phase to the existing differences in the tire tread circumferences of the individual wheels in response to the distance covered and in consideration of the respective driving situation.
Experience has shown that drivers cannot be expected in each case to reliably operate the calibration tip switch. Therefore, an object of the present invention is to relieve the driver of the obligation of reliably operating the calibration tip switch and to find a most easy way, with least possible effort, to start the learning phase in the necessary intervals or after the occurrence of defined events.